1. Field of the Invention
The present invention relates to a method of machining a substrate such as a semiconductor wafer provided at its face-side surface with an appendant part including protruding metals and a resin part, in which the appendant part is cut and the back-side surface of the substrate is ground to make the substrate thinner.
2. Description of the Related Art
The flip flop technology has been known as one of mounting technologies for the purpose of suppressing the volume needed to mount a semiconductor chip onto a wiring substrate or for the purpose of realizing a higher transfer speed of electrical signals. The flip flop technology includes forming protruding electrodes having a height of, for example, about 15 to 100 μm on a surface of a semiconductor chip, and bonding the electrodes directly to electrodes provided on the mounting substrate side. The protruding electrodes are formed by plating or the like method, and, if the plurality of protruding electrodes are nonuniform in height, the comparatively lower-height electrodes may fail to be bonded to the corresponding electrodes on the mating side. This problem becomes more conspicuous as the density of the electrodes arranged on the chip surface becomes higher.
Taking this into account, in order to satisfactorily bond all the protruding electrodes to the mating electrodes, it has been practiced to render the protruding electrodes uniform in height by collectively cutting their tips at the stage of the wafer not yet divided into the semiconductor chips (refer to, for example, Japanese Patent Laid-Open No. 2000-173954). After the cutting on the face side of the wafer is conducted to make the protruding electrodes uniform in height, the wafer is thinned to a desired chip thickness by grinding the back-side surface of the wafer. For grinding the back-side surface of the wafer, there has been known a grinding apparatus for performing infeed grinding in which the wafer is suction held on a vacuum chuck type chuck table, with its back-side surface exposed, and, while the wafer is revolved by rotating the chuck table, a grinding tool such as grindstone is pressed against the back-side surface of the wafer (refer to, for example, Japanese Patent Laid-Open No. 2003-051473).
In forming the protruding electrodes by a plating method, a resist layer is formed on the face side of the wafer, exposure to light is conducted to expose only those portions of chip surfaces at which electrodes are to be formed, and a plating film is deposited on the wafer surface portions exposed by the exposure, i.e., in holes bored in the resist layer, whereby the electrodes are formed. Therefore, at the time of cutting the tips of the protruding electrodes as above-mentioned, the resist layer is also cut simultaneously. Conventionally, the back-side surface grinding for adjusting the wafer thickness to a desired value has been conducted either before the formation of the protruding electrodes or after the removal of the resist layer subsequent to the formation of the electrodes. In the former case, the back-side surface of the wafer is ground before forming the electrode by plating. Therefore, when the desired thickness is comparatively small, warping of the wafer may occur in the step of forming the electrodes by plating after the thinning, or breakage of the wafer may occur in the subsequent step of cutting the tips of the electrodes.
In the latter case, on the other hand, a protective tape is adhered to the face side of the wafer for the purpose of protecting the protruding electrodes (obtained by removing the resist layer) when the face side of the wafer is sucked onto the above-mentioned chuck table or the like. While such a protective tape is also applied for protecting the electronic circuits formed on the face side of the chips in the case of grinding the back-side surface of the wafer before forming the electrodes, in the case of grinding the back-side surface after electrode formation, a protective tape comparatively thick and rich in elasticity is used for effectively damping the machining load exerted by the grindstone at the time of grinding and for preventing the electrodes from being damaged due to contact with the chuck table.
However, when the wafer with such a protective tape adhered to the face side thereof is subjected to the back-side surface grinding by the above-mentioned infeed grinding, the machining load exerted on the wafer increases along the direction from the inner peripheral side toward the outer peripheral side (because the circumferential velocity of the revolving wafer is higher on the outer peripheral side), and, therefore, the protective tape elastically deformed so as to be compressed in thickness under the machining load similarly exerted thereon is more compressed to be thinner on the outer peripheral side. Thus, the machining load tends to be consumed in compressing the protective tape rather than in the intended grinding, and this tendency is more conspicuous on the outer peripheral side; as a result, the wafer having been subjected to the back-side surface grinding would be thinner in an inner peripheral part thereof and thicker on the outer peripheral side. When the wafer is thus nonuniform in thickness, there will be a problem, even though the electrodes are uniform in height for example, that the total thickness of the wafer is nonuniform, i.e., the heights from the back-side surface to the top parts of the protruding electrodes in a single wafer are nonuniform.